Can somebody explain the appeal to me? Not trying to start a flame war, just trying to understand.

A 24mm pancake (which won't have IS) seems a bit redundant with the 24mm 2.8 IS, which by all accounts is a very good lens, has USM and which Canon cut the price to a much more reasonable level. The size seems kind of irrelevant once you put it on a 5D, 6D or other full frame body and for an SL1, you end up with a 37mm lens which is barely in the wide-angle realm.

Do people like these pancakes just because they are cute (no argument there)? What am I missing?

Size size size. It makes your rig so small you don't mind leaving the bag behind and just slinging your camera around your neck all day. Or it's such a small item that it's a no brainer to throw it in your bag as another FL option.

There's also a side argument (that some would refute) that the smaller your entire rig is, the more likely you'll bring it at all to take pictures.

But it will never 'compete' head to head at a feature level with larger lenses that offer IS, USM, weather-sealing, mechanical manual focusing, etc.

So I see pancakes as a nice option to reduce size when you don't need all those features -- leisure, walkaround, and street come to mind for lenses like these.

Size size size. It makes your rig so small you don't mind leaving the bag behind and just slinging your camera around your neck all day.

I leave my 70-200/2.8 "around my neck" all day, no problem. It's all about placement (shoulder, not neck), and strap (Optech, not Canon).

^^ This a dozen times. I already throw my 28 1.8 in a bag without thinking about it... compared to a 70-200 2.8, which I'm totally comfortable with, any of those USM consumer primes (28, 50 1.4, 85 1.8 gets a tad long with hood, but still not bad) are really pretty portable...

Do people like these pancakes just because they are cute (no argument there)? What am I missing?

Many people bought the 40 mm just because it was 'cool' and then lost it somewhere between the padding of their camera bags and never realized it went missing

Funny idea! Just checked if my copy of the 40mm is in the lens drawer where it has to be ... it was there so I haven't lost it in the padding of my back packs

But a real advantage is that at least an APS-C body with that lens fits in a lens compartment so it has solved the problem of carrying two bodies in a not too large photo back pack.

I tend to use 150mm equiv as MY standard lens. The 64mm equiv of this lens on APS-C is a moderate wide angel for ME. Two bodies with 100mm Macro and 40mm pan cake are a good combo for ME.

About a 24mm pan cake: I don't believe that it is possible to produce one at f/2.8 for full frame. Might be an EF-S - so not too interesting for me. I own the old EF 2.8/24mm and this one is at least very light and on the compact side.